1. Field of the Invention
The present invention relates to a hydraulic controller for a hydraulic actuator incorporated into a power transmission mechanism for switching between on and off states of a power transmission.
2. Related Background Art
Conventionally, there has been known a vehicle that uses one of front and rear wheel pairs as main driving wheels driven by a main driving source such as an engine and the other as auxiliary driving wheels driven by an auxiliary driving source such as an electric motor, in order to drive the auxiliary driving wheels when starting on a snowy or punishing road to assist the start and to regenerate power by the electric motor, which is the auxiliary driving source, at deceleration. In this case, a power transmission mechanism, which couples the auxiliary driving source to the auxiliary driving wheels, incorporates a hydraulic actuator such as a hydraulic frictional engaging component (a hydraulic clutch or a hydraulic brake) or the like for switching between on and off states of a power transmission. Thereby, a power loss is prevented from being caused by a reverse drive of the auxiliary driving source by putting the vehicle into the on state of the power transmission only when necessary such as at start or deceleration and putting the vehicle into the off state of the power transmission (disconnecting the auxiliary driving source from the auxiliary driving wheels) under normal conditions.
Conventionally, there has been known a hydraulic controller for the foregoing hydraulic actuator, comprising a hydraulic circuit arranged to supply oil from a hydraulic pump to the hydraulic actuator via a regulator valve, a one-way valve, and a control valve for controlling an operation of the hydraulic actuator, wherein an accumulator capable of accumulating a hydraulic pressure necessary for the operation of the hydraulic actuator is connected to an oil supply passage connecting the one-way valve and the control valve of the hydraulic circuit (Refer to, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2003-54279). In this technology, if the oil supply passage contains oil due to an action of the one-way valve and the hydraulic pressure of the oil supply passage (the hydraulic pressure of the accumulator) is higher than or equal to a predetermined pressure, the hydraulic actuator is operable without driving the hydraulic pump. Therefore, the hydraulic pump is driven only when the hydraulic pressure of the oil supply passage is lower than the predetermined pressure to start accumulation to the accumulator, while the hydraulic pump is stopped under normal conditions to reduce energy consumption caused by driving the pump.
Incidentally, to lubricate the power transmission mechanism by using the hydraulic pump, an oil passage for lubricating the power transmission mechanism is sometimes connected to the regulator valve in the drain side thereof. In this case, to improve the lubricating performance of the power transmission mechanism, it is desired to drive the hydraulic pump regularly to constantly supply the lubricating oil.
In this regard, a load pressure equal to a setting pressure of the regulator valve is applied to the hydraulic pump. In the foregoing conventional example, the setting pressure of the regulator valve is set at a considerably high hydraulic pressure where the accumulator is full. Therefore, to supply the lubricating oil constantly in the conventional example, the hydraulic pump need be regularly driven under the application of the high load pressure. This increases the energy consumption caused by driving the hydraulic pump, thus deteriorating a fuel efficiency.